For boiling-water nuclear power plants (BWR power plants), some water-level meters for use to monitor water levels in a reactor have been proposed. For example, a monitoring apparatus for measuring a water level in a reactor core has been proposed (e.g., Patent Document 1). The monitoring apparatus described in Patent Document 1 is configured to use gamma-ray heating in a reactor, insert a sensor structured by combining a thermocouple and stainless steel rod into the reactor, and continuously monitor the reactor water level by detecting an AC component and DC component of output.